1. Technical Field
The present invention relates to a screen unit.
2. Related Art
A screen unit which includes a substantially rectangular screen member having flexibility, a winding shaft for supporting one end of the screen member and functioning as a shaft around which the screen member is wound, a support body for supporting the side of the screen member opposed to the side supported by the winding shaft, and an elevating mechanism attached to the support body to control the height of the screen member is known (for example, see JP-A-2000-131766).
In addition, a screen unit which includes a screen, an upper end support member for supporting one end of the screen, a base member to which a winding member is attached, the winding member for supporting the other end of the screen with a winding force applied thereto, and a connecting mechanism (link mechanism) extended between the upper end support member and the base member and having an upper arm and a lower arm pivotally connected to each other is known (for example, see JP-A-2004-46078).
According to the screen unit disclosed in JP-A-2000-131766, the elevating mechanism which has three pairs of arms connected with one another is extended between the support member for supporting one end of the screen member and the winding shaft for supporting the side of the screen member opposed to the side supported by the support body. A pair of springs (gas springs or coil springs) are disposed between the one pair of the arms positioned at the center of the elevating mechanism in the up-down direction and the one pair of the arms positioned on the winding shaft side. According to this structure, the elevating mechanism is stretched and contracted by using the pressure or the elastic force of the springs to achieve expansion and storage of the screen member.
In addition, in case of the screen unit shown in JP-A-2000-131766, expansion of the screen member is also achieved by using tension applied to the screen member as well as stretch and contract of the elevating mechanism by using a pair of the spring coils.
According to the screen unit disclosed in JP-A-2004-46078, an urging unit (gas spring) for urging the lower arm of the connecting mechanism to shift upward is equipped. In this structure, the expanded condition of the screen can be maintained by producing a balanced condition between the downward winding force applied to the screen by the winding member, the upward shifting force applied to the arm of the connecting mechanism by using the gas spring, and others.
According to the screen unit disclosed in JP-A-2000-131766, the surface (rear surface) opposite to the surface (front surface) to which the screen member is attached with respect to the elevating mechanism is not tensioned. Thus, the force on the front surface side of the elevating mechanism is not balanced against the force on the rear surface side. In this case, the elevating mechanism is difficult to be maintained in a vertical posture, and inclined toward the side to which the screen member is attached. As a result, the screen member itself is inclined, causing deformation of images projected thereon from a projector.
According to the screen unit disclosed in JP-A-2004-46078, the front surface side to which the screen is attached with respect to the connecting mechanism is tensioned by the winding force applied to the screen under the expanded condition of the screen, but the rear surface side is not tensioned. In this condition, the force on the front surface side of the connecting mechanism is not balanced against the force on the rear surface side, which produces the possibility that the connecting mechanism is inclined toward the front surface side. In this case, the screen itself attached to the front surface side of the connecting mechanism is inclined, causing deformation of images projected on the projection surface of the screen from a projector or the like.